Re: [PATCH 02/10] struct ref_transaction: add a place for backends to store data

[Michael Haggerty <mhagger@alum.mit.edu>]

On 09/08/2017 09:02 AM, Jeff King wrote:
> On Tue, Aug 29, 2017 at 10:20:26AM +0200, Michael Haggerty wrote:
> 
>> `packed_ref_store` is going to want to store some transaction-wide
>> data, so make a place for it.
> 
> That makes sense, although...
> 
>> diff --git a/refs/refs-internal.h b/refs/refs-internal.h
>> index b02dc5a7e3..d7d344de73 100644
>> --- a/refs/refs-internal.h
>> +++ b/refs/refs-internal.h
>> @@ -242,6 +242,7 @@ struct ref_transaction {
>>  	size_t alloc;
>>  	size_t nr;
>>  	enum ref_transaction_state state;
>> +	void *backend_data;
>>  };
> 
> This is just one pointer. Once we start layering ref backends (and
> already we're moving towards a "files" layer which sits atop loose and
> packed backends, right?), how do we avoid backends stomping on each
> other (or worse, dereferencing somebody else's data as their own
> struct)?

My conception is that layered backends would be separated as much as
possible, and if the "top" layer needs to modify the "bottom" layer, it
would do so via a separate reference transaction on the bottom layer.
That transaction would be owned by the bottom layer, which would be able
to use the corresponding `backend_data` pointers however it likes.

You can see an example of this construct in patch 08.

Michael